Inability to de-op players if listed in ops.txt with non-lowercase letters

Summary

### Impact Originally reported in iTXTech/Genisys#1188 ```txt PotterHarry98 potterharry98 ``` `deop PotterHarry98` will remove `potterharry98` from the ops.txt but not `PotterHarry98`. Operator permissions are checked using `Config->exists()` with `lowercase=true`, which will result in a match: https://github.com/pmmp/PocketMine-MP/blob/22bb1ce8e03dba57173debf0415390511d68e045/src/utils/Config.php#L449 This means that it's possible to make yourself impossible to de-op (using commands) by adding your name to ops.txt with uppercase letters. ### Patches 4d37b79ff7f9d9452e988387f97919a9a1c4954e ### Workarounds This can be easily addressed by removing the offending lines from ops.txt manually. ### For more information If you have any questions or comments about this advisory: * Open an issue in [pmmp/PocketMine-MP](https://github.com/pmmp/PocketMine-MP) * Email us at [[email protected]](mailto:[email protected])